I further characterized the purine degrading pathway of Methanococcus vannielii. The first three steps are identical to those described for clostridia. However, the final enzyme in the clostridial purine metabolic pathway, glycine reductase, was not found in M. vannielii. I purified xanthine dehydrogenase from M. vannielii. The native enzyme has a molecular weight of 230,000 with subunits of molecular weights of approximately 85,000 and 31,000. Additionally, I purified carbon monoxide dehydrogenase from M. vannielii, and determined it to be a protein of molecular weight 220,000 with subunits of molecular weights of 89,000 and 21,000. I discovered that the previously unseen CODH-catalyzed reduction of carbon dioxide could be driven by the reduced form of a low potential viologen. Additionally, I found that CODH could catalyze c-14 exchange between carbon dioxide and carbon monoxide.